1. Field of the Invention
This invention pertains to the field of miniature electric components such as miniature computer capacitor chips and resistor chips used in various circuitry in the computer industry. These are called various names, such as "chips", "integrated passive components", "surface mount components", "array chips" and the like. More particularly, this invention pertains to a novel means of handling these miniature components for such operations as performing visual surface inspections that are required to sort them according to pass/fail visual inspections and electrical tests.
2. Description of the Prior Art
Developments in the computer industry continue at an unabated pace. Computers continue to grow in importance in our daily lives almost, it appears, to the point of dominating them. Computers reach all ages and areas of activity. Infants are exposed to computers through computer games that teach musical tones and image recognition. Computer games are available for preschoolers that lay the basis for enchancement of math, reading and geography skills. Elementary, junior high, high school and college students take classes in computer theory, programming and operation. For those who entered the work force before computers became a part of daily life, seminars, home-study courses and public television courses are offered to introduce the use of computers, computer operation and software opportunities. Every business in the United States of America uses computers somewhere in its operation.
The increased use of computers in business and education has created a demand for more computer capacity and faster processing times. It seems that each year, computers are made with greater performance and memory. Only a few years ago, computer memory was measured in megabytes whereas, today, it is measured in gigabytes. In addition, other electronic devices, such as VCRs, television sets, cameras, camcorders, auto alarms, radios and the like are all using more and more computer hardware to expand their capabilities. Competition among various electronic companies has forced computer and computer-accessory manufacturers to design more powerful and more capable circuitry.
At the center of all this activity are electronic components so small that one could easily overlook them. They are miniature capacitors and like devices, of a general size of 0.040 inches long with a square or rectangular cross-section 0.020 inches on a side, with spaced-apart, flat, end surfaces. These miniature capacitors are constructed of multiple layers of electrically conductive material held apart by minute layers of a ceramic dielectric that are assembled and fired to produce a tiny, ceramic device much smaller than a grain of rice. They are used by the thousands in computer circuitry. As such, they are in great demand. There are resistors of the same general size and shape that are included in the generic term "chips". Even more astonishing is the fact that some of these chips are now being made in multiple units, still smaller than a grain of rice, that have segregated conductive end surfaces. In computer circuitry, these chips are soldered directly to the computer circuit board on their flat side surface so that soldering connecting wires to the circuit board is eliminated.
These devices are so small that hundreds of them are contained in a teaspoon full. They are so small as to be incapable of visual inspection except under an intense magnifying lens, such as a microscope. In forming the solderable connections of this minute chips, the specifications call for accuracy in the order of a very few thousandths of an inch. Surface tension and other physical chemistry phenomena cause flaws to be developed in the surfaces of the chips that can render the chips unusable provided the flaw can be observed in time to remove the chip from further processing. For the special soldering techniques required on these minute chips, the conductive ends of the computer capacitor chips are coated with a silver paste, dried, and later fired at high heat to cure or set the silver paste. The conductive ends are thereafter treated with other materials, such as nickel plating, and the like, to make them amenable to being soldered to a specially designed copper "flat" or "trace" located on the computer circuit board. To apply this silver paste (called "terminating") to the computer capacitor chips, certain inventions have been made, in hand-operated tools and in machine-handling mechanisms, with which to position the chips for coating.
But metalizing and curing these computer capacitor chips is only part of the process. Before using them in any circuitry, each chip must be visually inspected for surface flaws and further tested to determine if it is of a capacitance value usable in a specific electric circuit and if it possesses sufficient other electrical and physical properties that will allow it to withstand the rigors of electrical operation. Further, each chip must be sorted into groups of pass/fail visual inspection and specific ranges of electrical properties so that they can be used most effectively in electronic circuitry.
Because of imperfections in materials and the multi-step nature of the chip-building process, each chip that has already been examined and found to be without surface flaws, still winds up with certain "parasitic" qualities, i.e., impedance qualities in addition to capacitance. Each parasitic quality affects and modifies the chip's capacitor characteristics. Accordingly, it is important to determine the nature and value of these parasitic qualities before inserting the chip into the electrical circuit.
The property of a capacitor that limits the flow of alternating current is called its reactance (X.sub.c) and is measured in ohms. The term impedance (Z), also measured in ohms, includes the effect of ordinary ohmic resistance as well as reactance.
"Capacitance" is simply the ratio of the charge acquired (Q) to the applied voltage (V) for any given pair of conductors that are near one another. More specifically: ##EQU1## A capacitor has a capacitance of one farad (F) if one coulomb of charge causes a potential difference of one volt. For most computer applications, however, a chip is measured in microfarads (1 .mu.F=10.sup.-6 F) or picofarads (1 pF=10.sup.-12 F). The test to measure the capacitance of the chip is called the "capacitance" (or CAP) test.
An ideal pure reactance dissipates no power; all energy used to charge the capacitor is recovered upon discharge. In the real world, however, there is always some associated resistance that dissipates some power thereby decreasing the amount of energy that can be recovered. A quality factor (Q) is used to describe a capacitors' purity. Q is 2.pi. times the ratio of energy stored to energy lost (over unit time), and is a unitless number. A Dissipation Factor (or Df) test is used to determine this property of the chip.
A "Flash" test is conducted on the chip for detecting internal flaws which are detrimental to the electrical integrity of the capacitor and cannot be found with normal capacitance and dissipation factor measurements. The most commnon surface flaws are cracks in the surface and smear or spill-over of solder from one conductor to another or to a spot on the surface that is designed to remain free of solder. The most common internal flaws take the form of irregular voids, cracks or open areas in the dielectric material that separates the conductors, embedded foreign material, thin spots in the dielectric or electrode, and poor contact between the electrodes and the terminiation paste. To uncover surface flaws, the chip is subjected to visual examination under a high-power microscope. To uncover internal flaws, the chip is typically subjected to a test voltage of more than twice its rated voltage, held (soaked) at that voltage for a short period of time, and any loss of voltage thereafter is measured.
The sequence of the "Flash" test is:
a. Part Present Test--verifies the capacitor is present and made good contact with the test probes. PA1 b. Charge--the capacitor is charged with a constant current to the proper stress voltage. PA1 c. Soak--the capacitor is held at this voltage for a short period of time. PA1 d. Test--leakage current through the capacitor is compared against a limit. PA1 e. Discharge--the capacitor is discharged at some constant current rate. PA1 f. Part Present Test--insures the capacitor did not open during previous testing. PA1 g. Check Test--performed because the capacitor may fail under the stress of the discharge as well as the charge itself. This test is a repeat of the previous test, only with a reduced voltage (usually the rated voltage). PA1 a. Station 1--Both CAP and Df PA1 b. Station 2--Flash PA1 c. Station 3--IR PA1 d. Station 4--Both CAP and Df (redundant)
The insulation resistance (or IR) is a measure of leakage current across the capacitor and is the product of the chip's resistance and capacitance. For example, a 1 .mu.F capacitor tested at 25 vdc with 1000 M.OMEGA. resistance, has an IR of 1000.OMEGA.-.mu.F, or a leakage current of 0.025 .mu.A. This test is usually conducted by charging the capacitor over a long period of time to insure a maximum charge, removing the charging electrodes, monitoring the discharge over a short period of time, and calculating the internal resistance of the chip by its rate of decay.
For efficiency in operation, these tests are usually performed sequentially as follows:
Also, since the soak time for the true IR test can be several seconds, there is often more than one station for the IR test, sometimes up to 10 charging stations and a single test station. This way, the chip can keep moving along with other chips through the test machine, and still be subject to the IR test, without slowing down the entire testing procedure.
Each of these tests requires physical contact with the conductive ends of the chips. In some tests, the contact is single and only momentary. In other tests, the contact is multiple and/or prolonged. Prior to these required tests, others have invented a machine to conduct tests upon those chips and sort them pursuant to their test results, reference U.S. Pat. No. 4,406,373. This patented machine relies on the process of placing the chips in a planar carrier and arranging the holes in straight files and straight rows. The carrier is first laid flat and loose chips cast over it, and the bank subjected to vibration to urge the chips into the holes. The chip-filled carrier is then raised up and placed on a trolley at an oblique angle, and indexed past a plurality of test probes. The chips rest against, or abut, a conductive layer and the probes are advanced into contact with only one end of the chip, the other end being treated in common with other chips through the conductive layer. Once the row of chips has been tested, the carrier passes into contact with a set of receptacles where the values assigned to a particular chip during testing finds a corresponding receptacle having a range of values into which the chip's value fits, whereupon the chips are blown by compressed air out of their respective holes and into a particular receptacle.
The problem with this machine is that it is a batch process and does not have the capacity to rapidly test and sort the large quantities of capacitor chips needed in today's markets. In addition, the tests that can be conducted with this machine are limited and cannot involve all of the tests now needed on modern chips. The patented machine has been modified to run by robotics; however, it remains a batch process with limited testing, high labor costs, and low output. In this patented machine with its robotic improvements, significant time is lost in loading the planar carrier with chips and transferring them to the machine to begin the testing operation, as well as removing the empty carrier and relocating it to a remote area for reloading with fresh chips. This lost time has become an important factor of late where higher and higher throughput rates are demanded in the industry.